This is an amended New Investigator R0-1 application designed to test the hypothesis that neuropsychological tests can aid in the differentiation of clinically diagnosed dementia with Lewy bodies (DLB) and clinically diagnosed Alzheimer's disease (AD). Studies of the neuropsychology of DLB have been primarily retrospective, but suggest a different pattern of performance than AD. A Consortium on dementia with Lewy bodies has identified core clinical features necessary for a diagnosis of DLB, and use of this criteria provides for greater reliability of antemortem diagnosis. Making an early diagnosis is important for purposes of treatment, predicting course and for safety and optimization of function. Participants will include 100 patients with clinically diagnosed DLB, and 100 patients with clinically diagnosed AD. Secondary aims include determining whether clinical features are associated with distinct patterns of cognitive performance in DLB and whether there are differences in disease course of DLB relative to AD. New interview measures of visual hallucinations and fluctuating cognition, clinical features considered to be the hallmark of DLB, will be evaluated for psychometric properties and to obtain a finer characterization of the type, severity and persistence of these features.